Acousto-magnetic technology has been widely used in electronic article surveillance (EAS) for over two decades. The original U.S. Pat. No. 4,510,489, issued on Apr. 9, 1985, to Philip M. Anderson, III, disclosed that some amorphous ribbons have a rather high magneto-elastic coupling factor resulting in a strong resonating signal. This principle was utilized to make commercial anti-theft systems, for example, the anti-theft systems in supermarkets. An acousto-magnetic anti-theft system includes a detection device, deactivation device and deactivation verifier, AM tags, etc. A widely used commercially available detection device is the Ultramax™ detector made by Sensormatic Electronics Corporation. The Ultramax™ detector emits a 58 kHz pulse wave which excites active acousto-magnetic tags, leading to strong 58 kHz resonating signal that can be detected by the pick-up coils in the Ultramax™ detector. The signal is then amplified and analyzed to trigger an alarm. Deactivation is carried out by demagnetizing the bias in the acousto-magnetic tags, which will shift the resonating frequency of the acousto-magnetic tag out of the detection window, while significantly reduced the resonating amplitude. Therefore, the alarm will not be set off from a deactivated tag.
There are two types of anti-theft acousto-magnetic tags: anti-theft acousto-magnetic hard tags and anti-theft acousto-magnetic labels of tags. Anti-theft acousto-magnetic hard tags use amorphous ribbon as resonators and use permanent magnetic materials (such as bonded ferrite magnets) as bias. This type of anti-theft tag (such as Supertag™ I, II, III made by Sensormatic) can not be deactivated by the deactivators and are used inside stores repeatedly. The hard tag pinned on a soft item can be detached by a detacher after this item has been paid for, so that the paid item will no longer set off alarm at the store gate detection apparatus when the item is removed from the store.
Acousto-magnetic labels also use amorphous ribbon as the resonators, but conventionally use “semi-hard” magnetic material as the bias component. This type of acousto-magnetic labels can be repeatedly deactivated and re-activated. By demagnetizing the bias in the acousto-magnetic label glued on paid store item, the alarm will not be triggered when the item is removed from the store through the store gate detection apparatus. The bias is a key component of the acousto-magnetic label. The bias component will affect the resonating frequency to allow the detection system to differentiate the active or deactivated state of the acousto-magnetic labels. Meanwhile, the bias component significantly affects the performance and cost of producing the acousto-magnetic labels. Consequently, the development on the bias has been on-going internationally.
Since the granting of the original patent (U.S. Pat. No. 4,510,489), more patents related to the composition and methods of making new semi-hard bias materials have been granted, including U.S. Pat. No. 4,536,229 granted to Sungho Jin, et al on Aug. 20, 1985; U.S. Pat. No. 5,351,033 granted to Nen-Chin Liu, et al on Sep. 27, 1994; U.S. Pat. No. 5,716,460 granted to Neil R. Manning, et al on Feb. 10, 1998; U.S. Pat. No. 5,729,200 granted to Richard L. Copeland, et al on Mar. 17, 1998; U.S. Pat. No. 6,001,194 granted to Noriyuki Nakaoka, et al on Dec. 14, 1999; U.S. Pat. No. 6,181,245 granted to Richard L. Copeland, et al on Jan. 30, 2001; U.S. Pat. No. 6,689,490 granted to Hartwin Weber, et al on Feb. 10, 2004; U.S. Pat. No. 6,893,511 granted to Noriyuki Nakaoka, et al on May 17, 2005, and others.
“Semi-hard” magnetic materials have coercivity measured in direct current (DC) between the coercivity of soft and hard magnetic materials, which is to say that the coercivity of semi-hard magnetic materials is in the range of 10-300 Oe. The acousto-magnetic labels using a bias component formed of semi-hard magnetic materials with coercivity in the range of 56-90 Oe) will show higher stability in storage and transportation against disturbing magnetic fields in the environment. But this type of bias normally contains Cobalt (Co), which is a strategic material, or Nickel (Ni), which has a rising and fluctuating price over recent years, leading to higher cost of manufacture. For example, the earlier commercial acousto-magnetic labels used FeCrCo semi-hard bias for many years. That FeCrCo semi-hard bias contains expensive Cobalt (about 7-17% wt %).
Later, Vacuumshemelze (VAC) from Germany developed the SemiVac90™ bias formed of (FeCrCoNiMo) semi-hard bias with Hc in the range of about 70-80 Oe (as set forth in the aforementioned U.S. Pat. No. 5,729,200 and U.S. Pat. No. 6,181,245). SemiVac90™ contains lower quantities of Cobalt in the bias component, but still can not get rid of Cobalt or Nickel completely. Furthermore, Carpenter Technology Corporation (CarTech) in Reading, Pa., USA, developed a Co-free bias component MagneDur20-4™ (Fe-20Ni-4Mo, as described in the aforementioned U.S. Pat. No. 5,729,200 and U.S. Pat. No. 6,181,245). MagneDur20-4™ still contains relatively high Ni content (i.e. >8 wt %) and has a lower Hc measured at about 20 Oe. Meanwhile, VAC also developed Co-free bias Sensorvac™ (FeNiAlTi, see U.S. Pat. No. 6,689,490). However, Sensorvac™ still contains a high Ni content (8-25% wt %) and has a lower Hc at about 20 Oe.
In 1980, Dr. S. Jin from Bell Labs, USA, conducted lab studies on Fe—Ni and Fe—Mn alloy systems. (High-Remanence Square-Loop Fe—Ni and Fe—Mn Magnetic Alloys”, IEEE Transactions on Magnetics Vol. Mag-16 No. 5 September, 1980.) He pointed out that cold-drawn (>80% cold deformation amount) Fe-(8-16 wt %) Mn alloy wire (not strip though) followed by 500-550 C/aging 3.5 hours, then cold drawn again (>95% cold deformation amount) followed by 450 C/aging 10 min-2 hours) shows property combinations of Hc=28 Oe/Br=18000 Gs, Hc=85 Oe/Br=15000 Gs, Hc=240 Oe/10000 Gs. However, such magnetic property combinations (either Br is good but Hc is too low, or Hc is good but Br is too low) presented difficulties for practical technical applications, long after 1980, after such cold drawn alloy wire was investigated. Specifically, such Fe—Mn alloy wire found no applications in acousto-magnetic labels which were invented in 1982. It is worth noting that the wire and strip process methods (especially the technical difficulties in processing) for wire and strip are different.
In 1996, The Arnold Engineering, USA, disclosed a strip alloy Fe-(8-18 wt %) Mn, having a composition of Fe-12.9 wt % Mn-0.01 wt % Cr, as set forth in U.S. Pat. No. 5,716,460, through method similar to that taught by Dr. Jin, but with cold reduction rate at least 40%, then aging at least 30 min at above 400 C, cold reduction again but greater than 75%, then go through a final and necessary step in this invention, which is to strand anneal the cold rolled strip above 525 C (in fact it was 525-625 C) for less then 3 min to get a material with Hc at least 20 Oe, Br at least 8000 Gs. Although the Arnold method, i.e. disclosed in U.S. Pat. No. 5,716,460, suggested that the above processed material can be used as a bias in acousto-magnetic labels, the patent does not present any acousto-magnetic label embodiments in the specification or show any acousto-magnetic label protection parameters in the claims of U.S. Pat. No. 5,716,460.
Therefore, this '460 patent does not disclose if such manufactured material can practically make qualified acousto-magnetic labels with satisfied detection range and deactivation performance. Another problem of this '460 patent is the necessary key step for final strand anneal in a fairly low temperature, such as 525 C, and fairly short time, in less than 3 min. Such a manufacturing process would provide a rather inconsistent method of manufacturing in commercial production. In fact, the limited data listed in Table 1.1 of the patent already proves that only 1 min or only a 100 C temperature difference could result in 20% variation on Hc or Br. Such highly magnetic property fluctuated bias is unlikely practical, in contrast to very tight demands on consistent magnetic properties on bias materials, due to acousto-magnetic labels having a very narrow frequency detection window in detection systems (i.e., 57.8-58.2 kHz).
Both methods of manufacture proposed respectively in the Jin and Arnold patents employed an intermediate aging process (at least 30 min, preferred over several hours) at a dual phase zone, after first cold deformation. This would not be beneficial for massive production, because a dual phase temperature zone (400-600 C) is too low for effective hydrogen reducing atmosphere protection. Re-grinding such thin strip will cause high yield loss, offsetting the cost benefits by using Co-free and Ni-free or low Ni.
The Arnold production method, as well as all prior methods to process bias, first employed four high rollers to roll the strip down to about 0.2 mm, then use multiple rollers Z-mill (such as a high precision 20 or 26 roller machine) to finish rolling the strip to about 0.05 mm, which is the commonly accepted bias thickness in the acousto-magnetic industry. It is well known that low-cost four high rollers can only obtain strip thickness down to about 0.07 mm. To roll the strip down below 0.065 mm, a multiple roller has to be used, which significantly increases the manufacturing cost of producing the strip to far above the low priced material cost (such as Fe, Mn) of the strip. Thus, the manufacturing process high cost conflicts with the economic goal of reducing costs of large-scale commercially usable bias by using Co-free or Ni-free or low Ni materials.
Another consideration, since U.S. Pat. No. 5,729,200 disclosed decreasing the coercivity of the bias component to 20 Oe, the bias developments were limited to low Hc range. However, the original idea to reduce the deactivation peak field down to as low as 35 Oe in U.S. Pat. No. 5,729,200 was not realized practically. In fact the commercially used deactivators in current systems still were having peak deactivation fields as high as several hundred Oe to ensure rapidly deactivating labels from all orientations in complex environments at the check-out counter so as to avoid false alarms.
Meanwhile, the concept of “source tagging”, which is the application of acousto-magnetic labels on the products at the source of manufacturing, and then being transported by sea or land, to reach stores that can be many thousands of miles away from the manufacturing source, will have to keep a very stable magnetization state in the bias component to maintain the best activating state of acousto-magnetic labels. Some storage or transportation environments can be quite complicated, including utilization of an iron-based goods storage shelf, ferrous transfer rollers on conveyor belts, magnetic leaking field when large quantity acousto-magnetic labels are packaged together, security checking machines, different kind of electronic equipment and low frequency power line devices. Therefore, the acousto-magnetic labels made with low Hc bias are having difficulties to keep their stability. For instance, certain label supplier requires that label users avoid exposing the labels to an environment magnetic field over 8 Gs (Oe) in the datasheet. However, it is very difficult for label users to exactly determine whether their environments in storage or transportation are having such a weak magnetic field of 8 Gs (Oe). Label users have no responsibility or ability to control the environment magnetic fields.
Currently used low Hc semi-hard bias strip FeNiTiAl or FeNiMo with Hc=20 Oe all need strict controls on the aging temperature and time after cold rolling. The Hc is a rising process during the aging. Low Hc is at the range that Hc rapidly rises in the initial aging stage. Therefore, the low Hc is extremely sensitive to the aging temperature. It is easy to miss target low Hc window at 20 Oe, resulting in scraping of whole batches of final rolled strip due to over-aged Hc. Therefore, low Hc bias needs high precision production facility for manufacturing. In contrast, for high Hc (56-90 Oe) Fe-12Mn based bias strip alloy, the aging temperature range is quite wide, because the high Hc dependence on aging temperature becomes less and is flatter. It is now easy to produce large scale consistent and high quality strip for bias applications, greatly reducing the demand to the production facility precision and reduced the bias cost.
Another closely related issue is recognized by the current low Hc=20 Oe label manufacturers. If the total leaking magnetic field during the storage and transportation, resulting from packing large quantity AM labels made with low Hc bias, is above 10 Oe, these acousto-magnetic labels would start to be demagnetized by each other and lead to deterioration on label detection performance. Thus, to avoid such instability, the manufacturer uses an alternative magnetizing method to activate the acousto-magnetic labels and to control the label magnetizing directions as alternatively arranged as north and south poles, which is described briefly in U.S. Pat. No. 5,729,200, and more details on devices and methods in U.S. Pat. No. 6,020,817.
Such methods and devices increase manufacturing costs and further complicate the processes for manufacturing, storage and transportation of acousto-magnetic labels made with low Hc semi-hard bias. In contrast, by employing high Hc bias for AM labels, the labels will not be demagnetized even if they are all magnetized in same direction (or any other activating methods) and packed with thousands and ten thousands together. Therefore, the manufacturing, storage and transportation of acousto-magnetic labels can become simple and reliable.
An example of present acousto-magnetic label structure is shown in FIG. 3A in U.S. Pat. No. 6,359,563. The label structure consists of an elongated plastic housing and the cover on the housing. The cover can be made with cover film, double tape, semi-hard bias piece, and cover film, in the order from top to the bottom. At least one, or more, resonators are positioned inside the housing with the size thereof comparable to the cavity and arranged as layered structure. The shape of the bias can be a parallelogram or a parallelogram with corners being cut, or shaped in rectangular.
It would be desirable to provide a Co-free, Ni-free or low Ni (i.e. <8 wt %) high Hc bias, which can be consistently produced, along with an acousto-magnetic label made by this type of bias, to meet ever-growing demands on this product.